An engine is known that is provided with a liquid fuel tank for supplying liquid fuel such as gasoline and a gas fuel tank for supplying gas fuel such as LPG, and can use either one of the fuels in a selective manner. See JP60-21547U, for instance. This engine is also capable of changing the ratio of the two fuels depending on the prices and availability of the fuels.
In such an engine, to simplify the setting of the combustion condition, the engine may be configured such that only one of the fuels can be used at any given time. In such a case, it is necessary to shut off the fuel that is not selected from being supplied to the combustion chamber.
The supply of the undesired fuel can be shut off by providing a manually operated valve in each of the gas fuel supply passage connecting the gas fuel tank with the intake system of the engine and the liquid fuel supply passage connecting the liquid fuel tank with the intake system of the engine. The user opens the valve for the desired fuel while closing the valve for the undesired fuel. However, the user is required to operate the two valves, and this is not only inconvenient for the user but also creates the possibility that the user may fail to operate the valves as intended or required. Also, the valves are required to be free from leakage, and this adds to the cost of the engine.